With the advance of laser technologies such as a high-intensity, ultrashort pulse laser, a nonlinear optical effect such as the optical Kerr effect, which is brought about by a laser beam having intensity larger than that of normal light, of a material and various phenomena caused by the effect are becoming problems. That is, the nonlinear susceptibility of a material with respect to a higher-order term than the second order of an electric field is smaller than that for a first-order term, so only a linear response is observed for normal light. On the other hand, the above-mentioned effect of a second-order nonlinear term or higher appears for light, such as a laser beam, having a sufficiently large intensity (electric field).
Examples of this nonlinear optical effect are the self-focusing effect of light produced when a material is irradiated with large-intensity light pulse, and the self-trapping effect such as channeling or filamentation by which light propagates with its beam diameter kept small.